In electrodynamics and quantum electrodynamics, in addition to the global U symmetry related to the electric charge, there are also position dependent gauge transformations. Noether's theorem states that for every infinitesimal symmetry transformation that is local, there is a corresponding conserved charge called the Noether charge, which is the space integral of a Noether density. If this is applied to the global U symmetry, the result is the conserved charge where ρ is the charge density. As long as the surface integral at the boundary at spatial infinity is zero, which is satisfied if the current densityJ falls off sufficiently fast, the quantity Q is conserved. This is nothing other than the familiar electric charge. But what if there is a position-dependent infinitesimal gauge transformation where α is some function of position? The Noether charge is now where is the electric field. Using integration by parts, This assumes that the state in question approaches the vacuum asymptotically at spatial infinity. The first integral is the surface integral at spatial infinity and the second integral is zero by the Gauss law. Also assume that α approaches α as r approaches infinity. Then, the Noether charge only depends upon the value of α at spatial infinity but not upon the value of α at finite values. This is consistent with the idea that symmetry transformations not affecting the boundaries are gauge symmetries whereas those that do are global symmetries. If α = 1 all over the S2, we get the electric charge. But for other functions, we also get conserved charges. This conclusion holds both in classical electrodynamics as well as in quantum electrodynamics. If α is taken as the spherical harmonics, conserved scalar charges are seen as well as conserved vector charges and conserved tensor charges. This is not a violation of the Coleman–Mandula theorem as there is no mass gap. In particular, for each direction, the quantity is a c-number and a conserved quantity. Using the result that states with different charges exist in different superselection sectors, the conclusion that states with the same electric charge but different values for the directional charges lie in different superselection sectors. Even though this result is expressed in terms of a particular spherical coordinates with a given origin, translations changing the origin do not affect spatial infinity.
Implication for particle behavior
The directional charges are different for an electron that has always been at rest and an electron that has always been moving at a certain nonzero velocity. The conclusion is that both electrons lie in different superselection sectors no matter how tiny the velocity is. At first sight, this might appear to be in contradiction with Wigner's classification, which implies that the whole one-particle Hilbert space lies in a single superselection sector, but it is not because m is really the greatest lower bound of a continuous mass spectrum and eigenstates of m only exist in a rigged Hilbert space. The electron, and other particles like it is called an infraparticle. The existence of the directional charges is related to soft photons. The directional charge at and are the same if we take the limit as r goes to infinity first and only then take the limit as t approaches infinity. If we interchange the limits, the directional charges change. This is related to the expanding electromagnetic waves spreading outwards at the speed of light. More generally, there might exist a similar situation in other quantum field theories besides QED. The name "infraparticle" still applies in those cases.
